This invention relates generally to forgery-prevention and more particularly, it relates to a method of manufacturing documents, such as paper money, credit cards, identification cards, and the like, having fibers embedded therein to increase the safeguarding factor against forgery, and an apparatus for checking the authenticity of the documents. The fibers (or filaments) are coated with a magnetic, or magnetizable material, and the filaments are checked with respect to the correct type and density of their filiform particles.